The AR5 Synthesis Report has been published with all the usual rhetorics such as that we have only so much years left to act. Readers here know that my interest with regard to AR5 has been climate sensitivity. So let’s just shortly review what happened in the field of climate sensitivity between the Synthesis Report (SYR) of AR4 (2007) and that of AR5 (2014). Let’s focus on the SPM because this is what is supposed to be the most policy relevant information.

For GHG emissions scenarios that lead to stabilisation at levels comparable to SRES B1 and A1B by 2100 (600 and 850 ppm CO2-eq; category IV and V), assessed models project that about 65 to 70% of the estimated global equilibrium temperature increase, assuming a climate sensitivity of 3°C, would be realised at the time of stabilisation. [Figure SPM.8 on page 12]

The timing and level of mitigation to reach a given temperature stabilisation level is earlier and more stringent if climate sensitivity is high than if it is low. [page 20]

The best estimate of climate sensitivity is 3°C. [note d of table SPM.6 on page 20]

Equilibrium sea level rise is for the contribution from ocean thermal expansion only and does not reach equilibrium for at least many centuries. These values have been estimated using relatively simple climate models (one low-resolution AOGCM and several EMICs based on the best estimate of 3°C climate sensitivity) and do not include contributions from melting ice sheets, glaciers and ice caps. [note f of table SPM.6 on page 20]

The right-hand panel shows ranges of global average temperature change above pre-industrial, using (i) ‘best estimate’ climate sensitivity of 3°C (black line in middle of shaded area), (ii) upper bound of likely range of climate sensitivity of 4.5°C (red line at top of shaded area) (iii) lower bound of likely range of climate sensitivity of 2°C (blue line at bottom of shaded area). [Caption of figure SPM.11 on page 21]

Impacts of climate change are very likely to impose net annual costs, which will increase over time as global temperatures increase. Peer-reviewed estimates of the social cost of carbon23 in 2005 average US$12 per tonne of CO2, but the range from 100 estimates is large (-$3 to $95/tCO2). This is due in large part to differences in assumptions regarding climate sensitivity, response lags, the treatment of risk and equity, economic and non-economic impacts, the inclusion of potentially catastrophic losses and discount rates. [page 22]

Summarised: climate sensitivity and its uncertainties is highly relevant for the amount of future warming. The best estimate for climate sensitivity is 3°C, the lower bound is 2°C and the upper bound is 4.5°C.

Progress since the TAR enables an assessment that climate sensitivity is likely to be in the range of 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C.

Zero
Now straigth to the AR5 Synthesis Report SPM. It mentions this highly relevant parameter (according to AR4) … zero times! Not a word about it. The full Synthesis report does mention it four times. For example on page SYR-23 we read:

Climate system properties that determine the response to external forcing have been estimated both from climate models and from analysis of past and recent climate change. The equilibrium climate sensitivity (ECS) is likely in the range 1.5 °C–4.5 °C, extremely unlikely less than 1 °C, and very unlikely greater than 6 °C.

Now what has happened in the past seven years that climate sensitivity disappeared from the SPM of the Synthesis Report? Has it become irrelevant? Of course not. Climate sensitivity is all over the Synthesis Report because the models used to project the future climate have a climate sensitivity of about 3.5°C on average. So in all its projections IPCC assumes climate sensitivity is still >3°C. It’s there as some sort of hidden assumption.

Why not say so then? Well, exactly this assumption, that the model climate sensitivity is about 3.5°C, has been seriously challenged in the past few years in the scientific literature. The Lewis/Crok report A Sensitive Matter (published in March of this year) gave all the details about new observationally based studies that indicate the climate sensitivity is relatively low with best estimate values of between 1.5 and 2°C. Considerably lower than the 3.5°C climate sensitivity of the models.

Dilemma
Recently Lewis and Curry used all the relevant AR5 numbers and a very detailed uncertainty analysis to estimate the range and best estimate for climate sensitivity in a paper published in Climate Dynamics. Their preferred likely range is 1.25-2.45°C and the best estimate is 1.64°C. Again, these are not numbers invented by skeptics, those are the numbers of the IPCC itself. It assumes close to 100% of the warming since 1850 is due to humans, an assumption that goes much further than the iconic “it’s now extremely likely that most of the warmings since 1950 is due to humans” statement in AR5.

Now this specific paper of course came out after the IPCC deadline for the Synthesis Report. However as we document in the Lewis/Crok report, the IPCC was well aware of these recently published lower estimates of climate sensitivity. It lowered its lower boundary from 2°C back to 1.5°C (where it has been in most earlier IPCC reports).

The IPCC was saddled with a dilemma. A lot of conclusions in the report are based on the output of models and admitting that the models’ climate sensitivity is about 40% too high was apparently too…inconvenient. So IPCC decided not to mention climate sensitivity anymore in the SPM of the Synthesis Report. It decided to give the world a prognosis which it knows is overly pessimistic. One may wonder why. Did it want to hide the good news?

All the written submissions to the UK Energy and Climate Change Committee are now online. Many interesting things to read. Lots of critical submissions. Judith Curry calls Nic Lewis’ contribution a “tour de force” which it really is. Some readers here might know that Nic and I have been working for the past few months on a report for the Global Warming Policy Foundation about how AR5 dealt with climate sensitivity. His submission is a nice introduction/summary of our report which hopefully will be published in January. Below is Nic’s submission. I strongly encourage readers to read it in full.

Written evidence submitted by Nicholas Lewis

Credentials and statement of interests

I am an independent, self-funded climate science researcher. In recent years I have specialised in the key area of climate sensitivity. My work has been published in the peer reviewed literature and is cited and discussed in the IPCC’s Fifth Assessment Report (AR5). I was an expert reviewer of AR5.

Introduction and summary

The terms of reference for this inquiry ask various questions. I address the following questions; my related conclusions are italicised.

How robust are the conclusions in the AR5 Physical Science Basis report (AR5-WG1)?In the central area of climate sensitivity, they are misleading. The substantial divergence between sensitivity estimates from, on the one hand, satisfactory studies based on instrumental observations over an extended period and, on the other hand, from flawed studies and from computer models was not brought out.

Does the AR5 address the reliability of climate models?Not adequately. Shorter-term warming projections by climate models have been scaled down by 40% in AR5, recognising that they are unrealistically high. But, inconsistently, no reduction has been made in longer term projections.

Do the AR5 Physical Science Basis report’s conclusions strengthen or weaken the economic case for action to prevent dangerous climate change?Although the conclusions fail to say so, the evidence in AR5-WG1 weakens the case since it indicates the climate system is less sensitive to greenhouse gases than previously thought. Lees verder…

The UK Energy and Climate Change Committee invited anyone with interest in the AR5 report to submit answers on a long list of questions. The deadline has now passed and several people have already made their contribution public (Richard Tol, Paul Matthews, Mike Haseler). As sooner or later all the submissions will be public anyway I have decided to do the same. My submission follows below and can also be downloaded as a pdf here.

I am a Dutch freelance science writer based in Amsterdam. Since 2005 I specialised in the global warming debate. In 2005 as an editor of the Dutch monthly popular science magazine Natuurwetenschap & Techniek (recently this has become the Dutch edition of New Scientist) I published a long and critical article about the infamous hockey stick graph featuring the criticism of Stephen McIntyre and Ross McKitrick. Many of the issues described in that article came back in the Climategate emails.
I published a critical book in 2010 that focused on the third and fourth assessment reports of the IPCC (TAR and AR4). The Dutch Ministry of Infrastructure and the Environment then gave me funding to critically review AR5 as an expert reviewer.
Since Climategate I am in favour of a more constructive interaction between climate scientists with opposing views. Late 2012 the Ministry of Infrastructure and the Environment funded an international discussion platform, ClimateDialogue.org, that organises constructive dialogues between climate scientists with opposing views. This has been set up by the leading Dutch climate related institutes KNMI and PBL and myself. [1] We cover controversial topics and invite scientists with a range of views.
In 2013 I was co-author of my first peer reviewed paper (describing a European temperature shift in 1988).

How robust are the conclusions in the AR5 Physical Science Basis report?To answer this question is beyond the scope of this inquiry I would say. However your own introduction provides a good start to deal with it. You wrote: “The report concluded that, ‘it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.’ But it reduced the lower bound for likely climate sensitivity and for the first time did not publish a best estimate of it because of lack of agreement.”
It’s good that you picked up this apparent paradox. AR5 itself focused on the 95% certainty that humans are the cause of most (>50%) of the warming since 1950. Most media outlets brought this as the major news of AR5 writing things like ‘how much more certainty do you want (before you act)?’.
However this interpretation of the 95% claim is misleading. In a sense the 95% claim of AR5 (itself a result of expert judgment and not some sort of mathematical calculation) is a no-brainer.
To understand this we focus on this other important parameter, climate sensitivity (the rise in global temperature after a doubling of the CO2 concentration). Recently several papers have been published estimating climate sensitivity from observational data since 1850. These studies assume that almost all of the warming since 1850 is due to greenhouse gases. These papers then come up with best estimates for climate sensitivity in the range of 1.5 to 2.0°C, considerably lower than the best estimate of 3.0°C that IPCC has presented in all their assessment reports so far.
So claiming that at least 50% of the warming since 1950 is due to humans is meaningless. The much more important question is whether the contribution of greenhouse gases to warming is big or small. AR5 has all the ingredients to conclude that the contribution is much smaller than we have thought for the last three decades. But by not giving a best estimate for climate sensitivity it failed to communicate this important message. So IPCC failed to give policy makers its most important conclusion. And IPCC only dealt with this important decision in a footnote in the Summary for Policymakers (SPM).
The 95% claim also tells you nothing about the seriousness of the climate issue. The 95% can be completely in accordance with there being no climate problem at all. IPCC failed to explain this clearly and journalists didn’t pick it up.
To conclude: the 95% claim of AR5 has been misinterpreted by most people, including policy makers and the media as the final proof that we have a huge anthropogenic climate problem. The claim itself proves no such thing and is in fact pretty meaningless.
Although it seems contradictory, there is in itself no conflict between the increasing certainty (the 95% attribution claim) and not giving a best estimate for climate sensitivity (less certainty). The 95% claim is just very conservative and tells you little about the seriousness of the climate issue. Lees verder…

Roy Spencer and his colleague Danny Braswell have a new paper out claiming that climate sensitivity is very, very low: 1.3 degrees Celsius. Remember, the recently published WGI report of AR5 gave a range for Equilibrium Climate Sensitivity (ECS) of 1.5-4.5 and for the first time in its history IPCC did not give a best estimate for ECS. So Spencer’s new estimate lies outside the lower boundary of the IPCC AR5 range.

In the last report in 2007 their best estimate was 3 degrees Celsius. An ECS of 1.3 C by the way is very close to the theoretical radiative effect of a doubling of CO2 so it suggests that the positive feedbacks (mainly water vapor and clouds) that produce the 3 degrees of warming in GCMs are not taking place in the real climate.

Readers of this blog are probably aware that in the past year or so there have been a number of papers claiming that ECS is relatively low (between 1.5 and 2 C) based on observational data (Ring et al, Lewis, Aldrin et al, Otto et al, Masters). These studies use our best observational data for the period 1850-2013 (change in global average temperature, change in total forcing, change in ocean heat content). In doing this in practice they assume almost all the warming since 1850 is due to humans, for the simple reason that available forcing estimates for the sun over this period are very small. Note: these studies do take the recent “extra” heat in the oceans into account and therefore do not conflict with the recently popular hypothesis that try to explain the standstill in warming with heat going into the deep oceans.

Spencer and Braswell use a different period (1955-2010) and a different method than the above mentioned studies. They do use the same numbers for the total forcing though and the main reason their estimate for ECS is even lower is that part of their warming is “explained” by El Niño and La Niña events. They use a very simple 1D model and then look for the best parameter settings to describe the warming of the oceans in the period 1955-2010.

One of the most surprising things in the just released SPM is the absence of a best estimate for climate sensitivity. The SPM now says this:

The equilibrium climate sensitivity quantifies the response of the climate system to constant radiative forcing on multi-century time scales. It is defined as the change in global mean surface temperature at equilibrium that is caused by a doubling of the atmospheric CO2 concentration. Equilibrium climate sensitivity is likely in the range 1.5°C to 4.5°C (high confidence), extremely unlikely less than 1°C (high confidence), and very unlikely greater than 6°C (medium confidence)16. The lower temperature limit of the assessed likely range is thus less than the 2°C in the AR4, but the upper limit is the same. This assessment reflects improved understanding, the extended temperature record in the atmosphere and ocean, and new estimates of radiative forcing. {TFE6.1, Figure 1; Box 12.2}

16 No best estimate for equilibrium climate sensitivity can now be given because of a lack of agreement on values across assessed lines of evidence and studies.

So from a footnote we have to learn that no best estimate “can now be given because of a lack of agreement on values across assessed lines of evidence and studies”. How strange this is. Climate sensitivity is one of the most important parameters. It determines largely how much warming we can expect. If there is lack of agreement between different methods/studies, we want to know all about it. However, apart from this footnote, the SPM is silent about it. Hopefully the full report, which will be released on Monday, will give all the details.

Tradition
Ever since the Charney report in 1979, national and international reports about climate have given a best estimate for climate sensitivity. So to speak in IPCC terminology, it is unprecedented not to give one.

In most reports of the past several decades the best estimate was 3°C with a range of 1.5 to 4.5°C. Only in the first two IPCC reports the best estimate was 2.5°C. But a best estimate was always given. What made it so much more difficult this time that IPCC felt it was impossible to give one?

Good news
Here is my best guess. It is true that more methods are now available to estimate climate sensitivity. Traditionally the models (GCMs) had most weight in the value of climate sensitivity. However since early this century it has become possible to estimate climate sensitivity from observations as well. The method is very simple: one only needs the total amount of forcing increase, ocean heat energy increase and temperature increase between two periods. Several papers/letters have done this in the last year (Aldrin et al, Ring et al, Lewis, Otto et al) and they all conclude that climate sensitivity is somewhere between 1.5°C and 2°C.

This is really good news! Our climate definitely seems to be less sensitive than we thought for a long time. The range for climate sensitivity based on these observations is also much more constrained, somewhere between 1.2°C and 2.6°C. Note that the lower bound of this range falls outside the likely range that the SPM now gives of 1.5°C to 4.5°C.

The main reason that climate sensitivity has come down so dramatically is not the slowdown. It’s the fact that estimates for aerosol cooling have come down considerably since AR4 and as a result the total increase in anthropogenic forcing has increased considerably in only a few years. This was mentioned in the SPM:

The total anthropogenic RF for 2011 relative to 1750 is 2.29 [1.13 to 3.33] W m−2 (see Figure SPM.5), and it has increased more rapidly since 1970 than during prior decades. The total anthropogenic RF best estimate for 2011 is 43% higher than that reported in AR4 for the year 2005. This is caused by a combination of continued growth in most greenhouse gas concentrations and improved estimates of RF by aerosols indicating a weaker net cooling effect (negative RF). {8.5}

Now with considerably more forcing and no temperature increase, climate sensitivity has to come down! There is no other possibility. It is the only logical consequence. Unless…much more heat went into the ocean. Now the recent observationally based estimates for climate sensitivity take this into account. The increase of heat in the ocean is just by far not enough to compensate for the huge increase in the forcing.

Now why didn’t IPCC bring us this good news?IPCC reports rely for a large part on the climate models. All the claims about the future are fully or partly based on the GCMs. These models are also used to determine climate sensitivity. Now here comes the problem. The climate sensitivity of the CMIP5 models (used for AR5) is on average 3°C. Real world observations however indicate climate sensitivity is much lower, between 1.5°C and 2°C. Admitting that these observationally based estimates are more reliable, would be like admitting that the models are less reliable. This would then question all the projections that are mentioned in AR5.

That models are (probably) too sensitive for greenhouse gases is becoming clear already when we are looking at the recent past. Not at the slowdown of 15 years, no a full climatic period of 34 years. This was very well explained in Steve McIntyre’s latest blog article two minutes to midnight where he showed that over the period 1979-2013 models on average warm up 50% faster than the real climate.

The IPCC had an impossible task this week. Their models were already ‘disproven’ by the real climate before the report came out. They have been unable to explain why models warmed up 50% more than the observations show. And they couldn’t be fully transparant about this because then the report would be regarded as outdated at the moment of publication, including al their projections.

So they did the smartest thing they could do given the circumstances, avoid this very difficult issue by not giving a best estimate for climate sensitivity.

At the end of the press conference when co-chair Thomas Stocker was asked why they mentioned the 15 year slowdown at all if they think it was unimportant, he answered that IPCC wants to deal with difficult questions. However they avoided a much more crucial and difficult issue, climate sensitivity, and by doing this they left the policy makers in the dark.

I was interviewed by Skype yesterday by Jon Laurence of The Telegraph. The interview is now up at their website. Their introduction:

Science writer Marcel Crok says the 15-year slowdown in the expected rate of global warming shows the climate of the planet “is much less sensitive then we thought”.

The world’s foremost authority on the greenhouse effect, the Intergovernmental Panel on Climate Change (IPCC), will release its most comprehensive study to date on Friday in Stockholm.

The report is expected to say that the IPCC is now 95 per cent sure that humans are to blame for climate change, up from 90 per cent in 2007.

However, there has been a slowing in warming since 1998. The panel is expected to say this was caused by a temporary cooling cycle in the weather system and lower-than-expected solar activity.

Science writer Marcel Crok argues the Earth’s climate sensitivity – the estimate of how much the Earth’s climate will warm in response to the increased greenhouse effect – is less affected by human activity than the mainstream science community believes.

“Right now there is not really a good explanation for the slowdown, and this is really refreshing for the international debate because suddenly even mainstream climate scientists start to disagree with each other.

Note: I did not say that the lower climate sensitivity is due to the 15 year slowdown. The pause has not much influence yet on our observational estimates of Equilibrium Climate Sensitivity (ECS). This was shown for example in the Otto et al letter where the ECS for the most recent decade was slightly higher than for earlier decades (due to a more efficient heat uptake in the oceans). I did say that the slowdown is largely unexplained so far and that are currently many hypotheses around.

Tomorrow at 10 am Amsterdam time the SPM will be released. Leaks have been around for a while so everybody knows what’s in it. Things might have changed though.

I think one of the key issues in the SPM is climate sensitivity. I have been working on a reaction to AR5 together with Nic Lewis (we were both expert reviewers) which we can hopefully publish in the coming weeks.

So what did the final draft of the SPM say about climate sensitivity:

The equilibrium climate sensitivity (ECS) quantifies the response of the climate system to constant radiative forcing. It is defined as change in global mean surface temperature at equilibrium that is caused by a doubling of the atmospheric CO2 concentration. ECS is likely in the range 1.5°C to 4.5°C (high confidence), extremely unlikely less than 1°C (high confidence), and very unlikely greater than 6°C (medium confidence). The lower limit of the assessed likely range is thus less than the 2°C in the AR4, reflecting the evidence from new studies of observed temperature change using the extended records in atmosphere and ocean. {Box 12.2}

Most surprising is that they don’t give a best estimate. IPCC has always given a best estimate, sometimes of 2.5°C and other times of 3°C.

Met Office responded to David Rose’s latest contribution to the debate. In Rose’s article there is a box about Nic Lewis’ rather technical critique of the Met Office report on the pause, about which I blogged the other day. In their reply the Met Office notes that it “will require time to provide as helpful a response [to Nic lewis] as possible, so further comment will be released in due course. But right after that they claim:

The article states that the Met Office’s ‘flagship’ model (referring to our Earth System Model known as HadGEM2-ES) is too sensitive to greenhouse gases and therefore overestimates the possible temperature changes we may see by 2100.

There is no scientific evidence to support this claim. It is indeed the case that HadGEM2-ES is among the most sensitive models used by the IPCC (something the Met Office itself has discussed in a science paper published early this year), but it lies within the accepted range of climate sensitivity highlighted by the IPCC.

Equally when HadGEM2-ES is evaluated against many aspects of the observed climate, including those that are critical for determining the climate sensitivity, it has proved to be amongst the most skilful models in the world.

The information that the HadGEM2 model is too sensitive came from Nic lewis’ article. The red bar in the figure below is the ECS of HadGEM2. It’s clearly obvious that it’s higher than the ECS of all other models and even higher than the 95% upper range of the CMIP5 models.

Nic Lewis replied to the Met Office as follows:

I would like to comment on the statements:

“The article states that the Met Office’s ‘flagship’ model (referring to our Earth System Model known as HadGEM2-ES) is too sensitive to greenhouse gases…”

and (referring to the sensitivity of HadGEM2-ES):

“it lies within the accepted range of climate sensitivity highlighted by the IPCC.”

The IPCC stated in its 4th Assessment Report (WG1: Box 10.2): “we conclude that the global mean equilibrium warming for doubling CO2, or ‘equilibrium climate sensitivity’, is likely to lie in the range 2°C to 4.5°C.” It gave no other range for ECS in that report, nor has it as yet changed that range.

I therefore fail to understand how the Met Office can claim that HadGEM2-ES lies within the accepted range of climate sensitivity highlighted by the IPCC.

Nic Lewis

The Met Office would have done itself a favour if they had done their homework first before sending out another unjustified claim.

Boomerang
Why like a boomerang? In July the Met Office published three reports about the ‘pause’. In the third and I would say most important one they looked at the implications of the pause for estimates of climate sensitivity and projections of future warming. Their key conclusions were (see their concluding remarks):

Despite the fact that the first decade of the 21st century has been a period during which there was very little global mean surface temperature rise, the range of TCR [Transient Climate Response, MC] estimates from the CMIP5 models lies within the TCR derived from observations, including this period.

When projections from the newer CMIP5 models are combined with observations, and specifically including the surface temperatures from the last 10 years, the upper bound of projections of warming are slightly reduced, but the lower bound is largely unchanged. More importantly, the most likely warming is reduced by only 10%, indicating that the warming that we might previously have expected by 2050 would be delayed by only a few years.

In simpler words, Met Office claims the recent pause in global warming has had little impact on estimates of climate sensitivity and future warming and models and observations largely agree with eachother. Met Office backed this up with several figures. Below I show their figure 5, showing estimates for Equilibrium Climate Sensitivity (ECS), which is the warming due to a doubling of the CO2 concentration, after the system has reached a new equilibrium.

Met Office figure 5

Otto et al is a recently published study that estimated climate sensitivity from observations. Visually the message is clear: there is a huge overlap between Otto et al (observations), CMIP3 and CMIP5 (the models) and palaeo estimates. Models therefore are ‘consistent with’ the observations, which is important to have faith in their projections of future climate. Nic Lewis was a coauthor of the Otto et al paper together with a large group of lead authors of the upcoming AR5 report. Lewis wrote the response in a personal capacity.

Now to make a long introduction short, here is Lewis’ adjusted version of the Met Office figure 5:

Lewis added other recent estimates of ECS based on observations (Aldrin, Lewis, Masters), he removed the palaeo estimates (as these are far too uncertain and contain “little information”) and he used flasks to show the 5-95% range of the distribution. The black bars are Lewis’ best estimates (medians). The white bars are the Met Office best estimates, for CMIP3 and 5 Met Office used the means instead of the medians.

The “boomerang” is the red bar on top of the CMIP5 distribution. This is the ECS of Met Office’s own flagship HadGEM2-ES model. As one can clearly see Met Office’s own model is not only far more sensitive than the observations suggest but even more sensitive than all other models. As Lewis put it:

And HadGEM2-ES has an ECS that exceeds not only the 95% bound from Otto et al but also that from two other recent observationally-based studies. Moreover, both the TCR and the ECS of HadGEM2-ES exceed the 95% bounds derived not only from CMIP3 models but also from CMIP5 models other than HadGEM2-ES.

So the Met Office model’s best estimate lies outside the range of both the observations and all the other models. It is a clear outlier. Met Office failed to disclose this in their own report, only showing the range of all the models in their figure 5 (and in other figures).

Relevance for the upcoming AR5 report
Although Lewis’ response is directed at the Met Office report, his piece is also highly relevant for the upcoming AR5 report. In an article today in the Wall Street Journal Matt Ridley quotes from the leaked SPM of AR5 (my bold):

Specifically, the draft report says that “equilibrium climate sensitivity” (ECS)—eventual warming induced by a doubling of carbon dioxide in the atmosphere, which takes hundreds of years to occur—is “extremely likely” to be above 1 degree Celsius (1.8 degrees Fahrenheit), “likely” to be above 1.5 degrees Celsius (2.4 degrees Fahrenheit) and “very likely” to be below 6 degrees Celsius (10.8 Fahrenheit). In 2007, the IPPC said it was “likely” to be above 2 degrees Celsius and “very likely” to be above 1.5 degrees, with no upper limit. Since “extremely” and “very” have specific and different statistical meanings here, comparison is difficult.

Still, the downward movement since 2007 is clear, especially at the bottom of the “likely” range. The most probable value (3 degrees Celsius last time) is for some reason not stated this time.

Apparently IPCC is planning (countries will negotiate about the final text in Stockholm later this month) to lower its likely lower bound for ECS from 2.0 in AR4 to 1.5 in AR5. By doing this the median estimates based on observations shown in the Lewis’ figure above fall into the likely range. So this rightly reflects the recent literature.

However, less assuring is that Ridley reports that in the draft SPM “The most probable value (3 degrees Celsius last time) is for some reason not stated this time”. As can be seen clearly in the Lewis’ figure, all the recent most probable values based on observations lie between 1.5 and 2. The central estimate of the models is 3. Or as Lewis put it into his conclusions:

Observationally-based median estimates for TCR and ECS are often comparable to the bottom of model-based uncertainty ranges.

This means that only the least sensitive models come close to the observations. Now this generates a big dilemma for the IPCC authors which could explain why so far – in their draft SPM – they failed to mention a best estimate for ECS. Recently there has been is a growing discrepancy between observationally based estimates and model based estimates for ECS. Should IPCC give equal weight to both methods? Should IPCC average the central estimates of both methods (leading to a reduced new estimate of let’s say 2.5)? Or should they put most weight onto the observations? In the last case they should lower their best estimate to at least 2. That would be a spectacular result as the best estimates for ECS hardly changed at all since the 1979 Charney report.

I hope many countries (including my own) will urge the IPCC to publish a best estimate for ECS (like they did in AR4) or – in case not – to clearly explain why they decided not to mention it this time. I see this as one of the most if not the most important decision for the four day meeting in Stockholm. On 27 September we will know the answer.

Over at the Climate Dialogue website we start with what could become a very interesting discussion about the so-called tropical hot spot. Climate models show amplified warming high in the tropical troposphere due to greenhouse forcing. However data from satellites and weather balloons don’t show much amplification. What to make of this? Have the models been ‘falsified’ as critics say or are the errors in the data so large that we cannot conclude much at all? And does it matter if there is no hot spot?

The (missing) tropical hot spot is one of the long-standing controversies in climate science. In 2008 two papers were published, one by a few scientists critical of the IPCC view (Douglass, Christy, Pearson and Singer) and one by Ben Santer and sixteen other scientists. We have participants from both papers. John Christy is the ‘representative’ from the first paper and Steven Sherwood and Carl Mears are ‘representatives’ of the second paper.

Below I repost the introduction that we – the editors of Climate Dialogue – prepared as the basis for the discussion. Feel free to post it on your own blog with a link to the discussion at climatedialogue.org.

The (missing) hot spot in the tropics

Based on theoretical considerations and simulations with General Circulation Models (GCMs), it is expected that any warming at the surface will be amplified in the upper troposphere. The reason for this is quite simple. More warming at the surface means more evaporation and more convection. Higher in the troposphere the (extra) water vapour condenses and heat is released. Calculations with GCMs show that the lower troposphere warms about 1.2 times faster than the surface. For the tropics, where most of the moist is, the amplification is larger, about 1.4.

This change in thermal structure of the troposphere is known as the lapse rate feedback. It is a negative feedback, i.e. attenuating the surface temperature response due to whatever cause, since the additional condensation heat in the upper air results in more radiative heat loss.

IPCC published the following figure in its latest report (AR4) in 2007:

The figure shows the response of the atmosphere to different forcings in a GCM. As one can see, over the past century, the greenhouse forcing was expected to dominate all other forcings. The expected warming is highest in the tropical troposphere, dubbed the tropical hot spot.

The discrepancy between the strength of the hot spot in the models and the observations has been a controversial topic in climate science for almost 25 years. The controversy [i] goes all the way back to the first paper of Roy Spencer and John Christy [ii] about their UAH tropospheric temperature dataset in the early nineties. At the time their data didn’t show warming of the troposphere. Later a second group (Carl Mears and Frank Wentz of RSS) joined in, using the same satellite data to convert them into a time series of the tropospheric temperature. Several corrections, e.g. for the orbital changes of the satellite, were made in the course of years with a warming trend as a result. However the controversy remains because the tropical troposphere is still showing a smaller amplification of the surface warming which is contrary to expectations. Lees verder…